Firstly : I'd like to change the value of the image which has been clicked. But in the Javascript function named changeImage, :
'document.images[0].value' stays 'undefined' (even though I put some non-empty string in VALUE="")
So, how could I retrieve the value of images !!!

Secondly : Once the value of the selected image has been changed, I'd like to be able in the asp file to retrieve the value of the images too. Applying :
Dim str
str = document.images[i].value
====> doesn't work

So : If you've got a good experience in javascript and asp, and have any idea of answer, please, take a few seconds, write a thread to put me in the right way

The first thing you need to understand is the difference between Client Side script and Server Side script.

Client Side script is executed/interpreted by *YOUR* browser.

Server Side script is executed/interpreted by the *SERVER*.

Now, assuming that the *SERVER* is in HongKong and that, you the client, is on an airplane overseas with your laptop.

In order for something to happen on the *SERVER* the user must click or do something, in other words some sort of event must be triggered so that your browser makes a request to the *SERVER* in HongKong.

Now, let's get back to your problem...

My first question is:
You are using the <img> tag, but I don't see the src attribute. Shouldn't you have something like:

<img src="images/myimage.gif" width="15" height="15">

NOTE: I've assumed that you had an image called myimage.gif and that it was stored inside a folder called images.

Another thing, I've never used the onclick attribute of the <img> tag, why? because....I've always embedded my <img> tag between an <a href="">...</a> tag when I wanted an image to be clickable.

NOTE: I've assumed that your <form>...</form> name would be called frmMain and that you have a <hidden> field inside that <form>...</form> called hidField.
The xxxxxxxxx part is whatever you need to be put.